The present invention relates to an electronic engine control system for a lean burn engine and more particularly to an engine control apparatus capable of retrieving an optimum control parameter in accordance with an engine condition even if a dynamic range of a air-fuel ratio to be controlled is wide to control the engine.
In a conventional engine control apparatus, generally, control parameters such as a target air-fuel ratio, a target throttle opening and an ignition timing are determined from a two-dimensional data map having one axis in which an engine speed Ne is defined and the other axis in which a basic injection amount (injection time) TPbas calculated from the engine speed Ne and an actually measured intake air amount Qa is defined as described in JP-U-2-85843. Further, as a special example, as described in JP-A-6-129276, the control parameters are determined from a two-dimensional data map having one axis in which an engine speed is defined and the other axis in which a target torque calculated from an acceleration opening is defined.
The lean burn system functions to increase the combustion efficiency so as to effectively utilize energy contained in fuel so that the fuel consumption is improved. When the air-fuel ratio is set to be leaner than the theoretical air-fuel ratio, the fuel consumption rate is improved, although since the combustion is made unstable, various measures have been made for the drivability and the exhaust emission control.
Further, in the lean combustion, when the air-fuel ratio exceeds a certain value, the combustion is unstable and variation of torque is suddenly increased, so that the smooth driving is difficult. For this purpose, there has been proposed that accurate control of the air-fuel ratio is made in the lean area in order to suppress the variation of torque to an allowable value.
The above-mentioned two known examples of the method of setting the control parameters are now verified in the lean burn engine represented by an inner-cylinder injection engine in which the dynamic range of the air-fuel ratio to be controlled is wide.
In the calculation method of the control parameters described in JP-U-2-85843, if it is assumed that the air-fuel ratio is controlled to be fixed in all of the operation area, the intake air amount Qa is increased as a load is increased. Accordingly, since the basic injection amount TPbas is increased monotonously and corresponds to the torque in one-to-one manner, the control parameters can be set to the optimum value even for any torque if the basic injection amount TPbas is used in the control axis.
However, when the air-fuel ratio for a light load condition is set to be leaner than the air-fuel ratio for a heavy load condition, the intake air amount Qa is reduced as the load is increased and even when the load is heavy, there is an area where the basic injection amount TPbas is reduced. Accordingly, the basic injection amount TPbas does not correspond to the torque in one-to-one manner. Hence, when the basic injection amount TPbas is used in the control axis, the control parameters cannot be set to the optimum value for any torque. Further, the basic injection amount TPbas representing the load is desirably increased as the load is increased, while since there occurs the reverse phenomenon that even when the load is increased the basic injection amount TPbas is the same or even when the load is increased the basic injection amount TPbas is reduced, the control is remarkably unstable. Since the phenomenon appears remarkably as a difference for the set air-fuel ratio is large, the control is not materialized in the lean burn engine represented by the inner-cylinder injection engine having a wide dynamic range of the air-fuel ratio to be controlled.
Furthermore, in JP-A-6-129276, the control parameters are retrieved from the data map having one axis in which the engine speed is defined and the other axis in which the target torque calculated from the acceleration opening is defined, while it cannot be verified whether the actual torque meets the target torque or not. Accordingly, even if the optimum values of the control parameters are set for respective torques in the bench test using a dynamometer, a deviation between the actual torque and the target torque cannot be compensated in an actual vehicle which cannot obtain the actual torque and accordingly the optimum control parameters cannot be retrieved from the map.